1. Field of the Invention
This invention relates to an improved method of separating carbon dioxide from fluids, and more specifically, this invention relates to an improved method of separating carbon dioxide from flue gases that utilizes amine-based scrubbing and low regeneration temperatures of CO2 bearing mineral systems to permanently sequester carbon dioxide.
2. Background of the Invention
Increased global warming due to the presence and production of greenhouse gases such as carbon dioxide (CO2), makes the capture and permanent sequestration of carbon dioxide by economical means imperative.
Carbon dioxide capture is the separation of CO2 from emissions sources or from the atmosphere with the subsequent recovery of a concentrated stream of CO2 that is amenable to sequestration or conversion.
Pulverized coal (PC) plants, which are 99 percent of all coal-fired power plants in the United States, burn coal in the presence of air to create steam. The steam, in turn, drives a turbine to generate electricity. Carbon dioxide is emitted as part of the coal combustion flue gas at atmospheric pressure and at a concentration of 10-15 volume percent of the flue gas.
Post-combustion capture of CO2 poses a significant engineering challenge because (1) the low pressure and dilute concentration of the carbon dioxide dictate a high actual volume of gas to be treated; (2) trace impurities in the flue gas tend to reduce the effectiveness of CO2 absorbing processes; and (3) compressing captured CO2 from atmospheric pressure to pipeline pressure (1,200-2,000 pounds per square inch (psi)) represents a large parasitic energy load to the overall power generation process.
Analysis conducted at the National Energy Technology Laboratory (NETL) in Pittsburgh, Pa. and Morgantown, W. Va. shows that CO2 capture and compression using amines raises the cost of electricity from a newly-built supercritical PC power plant by 84 percent, from 4.9 cents/kWh to 9.0 cents/kWh.
The goal for advanced CO2 capture systems is that they increase the cost of electricity by no more than 20 percent compared to a no-capture case. Another goal is to have an absorbing moiety which does not undergo degradation as amines do in present capture systems.
U.S. Pat. No. 6,387,337 awarded to Pennline, et al. on May 14, 2002 discloses a carbon dioxide capture process using regenerable solid sorbents such as an alkali metal oxide or alkali metal carbonate. Potassium carbonate (K2CO3) or a similar moiety can be supported on a high surface area of activated alumina (Al2O3) (12 to 17 weight percent (wt. %) of potassium carbonate). These composite, solid sorbent materials are contacted directly with a CO2-bearing gas stream. The product formed from sorption of carbon dioxide is taken aside and heated to regenerate the sorbent. No significant chemical degradation is observed, but physical attrition can be significant. Physical attrition describes the separation of K2CO3 from the activated alumina support resulting from interparticle abrasion. Loss of K2CO3 from composite beads results in decreased CO2 bearing capacity of the beads. CO2 loading occurs between from about 60° C. to 100° C. according to the following reaction equations.H2O+CO2H2CO3  (1)H2CO3H++HCO3−  (2)K2CO3+H2O+CO22KHCO3  (3)
Subsequent thermal desorption of CO2 from potassium hydrogen carbonate (KHCO3)/alumina support takes place at temperatures around 145° C. to 150° C., according to the reverse of reaction 3, supra.
Aqueous scrubbing solutions containing monoethanolamine (2-aminoethanol) (NH2CH2CH2OH) (MEA) are also utilized for CO2 separation. The use of MEA solutions is energy-intensive, corrosive and follows the reaction equation infra2 MEA+H++HCO3−MEA−H++MEA.HCO3−  (4)
The regeneration of MEA scrubber solution has an enthalpy value for 20 and 30 mass percent solutions of +5308 kilojoules (kJ) and +4503 kJ per kilogram (kg) of carbon dioxide released, respectively. These enthalpies include solvent heating, vaporization of water, vaporization of absorbent, and the reaction enthalpy of Reaction 4. The regeneration also causes some degradation of the amine.
Other work has been done in the area of carbonate sorbents. Nelson, T. O.; Green, D. A.; Raghubir, P. G.; Portzer, J. W.; Coker, D.; McMichael, W. J.; and Figueroa, J., “Dry Regenerable Carbonate Sorbents for Capture of Carbon Dioxide from Flue Gas,” Fourth Annual Conference on Carbon Capture and Sequestration, Hilton Alexandria Mark center, Alexandra, Va., May 2-5, 2005. In this work, the investigators demonstrated the energy savings dry carbonate sorbents provide compared to conventional monoethanolamine (MEA) aqueous scrubbing solution systems.
In contrast to dry carbonate sorbents, the Benfield Process has found wide application for removal of acid gas components such as CO2 from mixed gas streams. The process entails the use of activated K2CO3 solutions (about 30 weight percent aqueous) for the absorption of carbon dioxide and thermal regeneration of the CO2 loaded solution by steam stripping. This process is designed for bulk CO2 removal from high pressure streams having a feed CO2 partial pressure of 6.8 atmospheres (atm) and, for CO2 removal, is typically used in synthesis gas treating in ammonia (NH3) plants and in direct iron ore reduction plants.
Enzymatic catalysis employing carbonic anhydrases, such as the biological catalysts (zinc metalloenzymes) responsible for interconversion of carbon dioxide and hydrogen carbonate (bicarbonate) (HCO3−) in living organisms, have been used to sequester carbon dioxide. For example, U.S. Pat. No. 7,132,090 awarded to Dziedzic, et al. on Nov. 7, 2006 discloses a method of converting carbon dioxide by placing the gas in a fluid medium, catalyzing conversion of the gas to carbonic acid with carbonic anhydrase, followed by reaction of the carbonic acid with a mineral cation such as calcium ion (Ca2+) or magnesium ion (Mg2+) to form a solid carbonate salt.
Studies have been made with regard to the removal of CO2 by metal oxides and metal hydroxides, respectively. Stolaroff, J. K.; Lowry, G. V.; and Keith, D. W., “Using CaO and MgO-Rich Industrial Waste Streams For Carbon Sequestration,” Energy Conversion and Management, 46, 687-699 (2005). In the first method, the industrial waste is mixed with water and reacted with CO2. A limitation of this method is the relatively low rate of reaction resulting from the limited solubility of CO2 in these mixtures.
The second method, described in: Stolaroff, J.; Keith, D., A Pilot-Scale Prototype Contactor for CO2 Capture From Ambient Air: Cost And Energy Requirements, GHGT−8; 8th International Conference on Greenhouse Gas Control Technologies, 19-22 Jun. 2006, Trondheim, Norway) allows for thermal regeneration of calcium carbonate by heating to temperatures of approximately 900° C.
The Solvay process utilizes two alkalis (including ammonia) in sequential order to convert CO2 to sodium hydrogen carbonate (NaHCO3) for commercial use, according to Equation 5.CO2+NaCl+NH3+H2O→NaHCO3↓+NH4Cl (I)  (5)
The Solvay process was not designed for CO2 sequestration and is not practical for use in the sequestration of CO2 from fossil fuel power plants. A study has been made where the Solvay process is modified to use an aqueous solution of a sterically hindered amine and a chloride to fabricate sodium hydrogen carbonate. Huang, H. P.; Shi, Y.; and Chang S. G., “Dual Alkali Approaches for the Capture and Separation of CO2,” Energy & Fuels, 15 (2), 263-268, 2001.
U.S. Pat. No. 6,969,418 awarded to Hu on Nov. 29, 2005 discloses a phase-enhanced gas-liquid absorption method wherein a binary organic-aqueous liquid system is used to capture CO2 and other acid anhydrides. A less dense organic liquid phase rests upon an aqueous phase and the organic liquid phase absorbs acid anhydrides at a faster rate and transports the acid anhydrides into the aqueous phase for subsequent reaction and eventual separation of CO2 in the form of reaction products.
U.S. Pat. No. 6,908,497 awarded to Sirawardane on Jun. 21, 2005 discloses a process for the manufacture of amine-containing solid sorbents whose absorption capabilities are independent of the sorbents' surface areas.
U.S. Pat. No. 6,547,854 awarded to Gray, et al. on Apr. 15, 2003 discloses a method for the manufacture of amine enriched solid sorbents for carbon dioxide capture.
None of the aforementioned patents or papers disclose a method to regenerate CO2-loaded aqueous amine solutions (MEA or similar amines) through reaction with a phase-separating reagent such as dissolved alkali metal carbonates, alkaline earth metal oxides, or alkaline earth metal hydroxides.
Also, none of the aforementioned patents or papers disclose a method to capture carbon dioxide via phase separation of dissolved CO2 as a crystalline solid precipitate. Furthermore, none of the aforementioned patents or papers disclose a method to accomplish rapid crystalline solid precipitate alkali metal bicarbonate formation through catalysis by MEA or similar amines. Also, none of the aforementioned patents or papers disclose a method to accomplish rapid alkaline earth metal oxide or alkaline earth metal hydroxide carbonation through catalysis by MEA or similar amines.
In addition, none of the aforementioned patents or papers disclose a method to use bases and base anhydrides present in industrial wastes as capture agents for CO2 by suspending the wastes in aqueous solutions of MEA or similar inhibited amines.
A need exists in the art for a method to reduce the energy cost of carbon capture. A need also exists in the art for a method to reduce the energy and financial cost of carbon dioxide regeneration from capture agents. The method should readily separate carbon dioxide in solid form.
A need also exists in the art for a method to sequester carbon dioxide using bases and base anhydrides present in industrial wastes through suspending the solid wastes in aqueous solutions.